This is the 315th most frequent Chinese word.
岁
"岁" means "year (of age)."
In this sentence, '岁' is used after a number to state a person's age.
我今年25岁。
I am 25 years old this year.
Here, '岁' describes the child's young age by following the number to denote years of age.
这个小孩只有五岁。
This child is only five years old.
In this instance, '岁' follows the number to tell the age of her dog, implying the longevity of the pet.